This invention relates to the art of drilling and in particular it relates to wire line core drilling apparatus.
In the course of wire-line drilling, the core barrel inner tube assembly is dropped or pumped along the bore of a drill string to a position just above or behind the drill bit. The drill string is provided with an annular landing shoulder therein. The inner tube assembly is also provided with a landing shoulder which is adapted to co-operate with and seat on the landing shoulder of the drill string. The inner tube assembly is provided with spring loaded latches which automatically move outwardly and engage in an annular recess, termed the latch seat, which is provided in the lower section of the drill string (otherwise known as the outer tube) thereby to anchor the inner tube assembly against axial movement in the bore. A drilling liquid, typically water, is pumped along the drill string thereby to propel the inner tube assembly along to the landing position which is of course correctly positioned relative to the drill bit.
The primary objective of a core drilling operation is to obtain a core of drilled material for purposes of geological analysis. The lower end of the drill string is accordingly provided with an annular drill bit of any desired well known variety, the bit having diamonds or boart embedded therein to enable the bit to cut through the hardest formations likely to be encountered. As the drilling proceeds, the rotating bit cuts through the formations and a core of the formation being drilled rises upwardly into and is captured by the core receiving barrel of the inner tube assembly. When the core barrel is filled, the drilling operator on the surface lowers an overshot assembly down the drill string by way of a wire line. The overshot assembly is arranged to engage with the upper end of the inner tube assembly and the wire line is then hoisted upwardly, in the course of which the spring loaded latches release thus allowing the inner tube assembly to be hoisted to the surface. The core of material, which has broken off from the formation, is captured within the inner tube assembly in well known fashion and when the inner tube assembly reaches the surface the core is removed and taken away for analysis. Following this, the inner tube assembly is then lowered down the drill string in preparation for the taking of a further sample. The movement is usually assisted by a flow of drilling liquid, i.e. water, this flow of water being provided by a flush pump which is capable of producing the flow rates and pressures required during the course of a drilling operation.
During the course of a normal core drilling operation, the above-noted pump forces the drilling liquid down the drill string, through and along the above-noted inner tube assembly, and downwardly to the bit where the liquid cools the bit and flushes away the cuttings therefrom, the fluid velocity being sufficient to lift these cuttings upwardly along the exterior of the drill string and along the drill hole up to the surface. The loss of drilling liquid circulation can give rise to serious problems. Since the liquid cools and lubricates the bit, loss of liquid will soon give rise to overheating and burning out of the bit in consequence of which the entire drill string must be pulled out of the hole. Even worse, loss of circulation may cause the drill string to cease up or jam within the hole owing to a build-up of cuttings between the drill string outer wall and the bore hole surface. (Drilling liquid can be lost when drilling through formations having substantial cracks and crevices. If the crevice is large enough, all of the liquid standing in the drill string and bore hole can be lost. When this happens, the pressure at the surface will drop to 0 and if there is an obstruction in the hole and the drilling liquid cannot pass through the bit because of bad rock conditions, by the time the pressure builds up to indicate this on the fluid gauge, it is likely too late, the bit will be either burned out and/or the drilling string will be stuck.) Removal of a stuck drill string can be a difficult operation and in serious cases the hole can be lost altogether.
Another problem with the system described above is that the drill operator on the surface often has difficulty determining when the latches are properly engaged in the latch seat. If the inner tube assembly is not properly landed on the landing rings and latched, and drilling is allowed to proceed, a great deal of time may be lost because the core then cannot be retrieved by means of the wire line system. The drill string must be moved from the hole and a fishing operation may be necessary to recover the core. At best, the core which is obtained is likely to be broken and unsuitable for an accurate analysis of same to be made.
In the past, the operator has used various inaccurate techniques to estimate when the inner tube assembly has landed and latched in position. A good operator should be able to listen to the descending assembly and may be able to hear the latches click into position. Other operators try to estimate when the correct position on the inner tube assembly has been reached merely by timing the descent of the assembly within the drill string. For these techniques to work, very experienced drillers are required and even then, problems are often encountered. An experienced driller will not have a sufficient "feel" for the situation and the resulting guess work can give rise to lengthy and costly delays.